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Skyway systems are predominately found in cities with cold climates, like the Minneapolis Skyway System:

Minneapolis Skyway System Representation

But why would a city with a warm climate have a skyway system when it is not necessary for comfortable transport?

Edit: Assume that the world is Earthlike and breathable. People can freely walk outside without being uncomfortable.

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    $\begingroup$ Does this planet, perhaps, have weather? $\endgroup$ – NomadMaker Jun 8 at 22:11
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    $\begingroup$ Keeps people off the street if they need to go from one building to another. There's something similar in Charlotte, NC and it is convenient/faster and reduces foot traffic congestion. And it often does provide nice coverage from the elements such as rain, occasionally cold weather, and hot weather. $\endgroup$ – Issel Jun 9 at 6:03
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    $\begingroup$ Please note that it is recommended to at least wait 24h before choosing an answer. This gives more people time to add their answer, which might have better or more complete answers you're not expecting yet. $\endgroup$ – Trioxidane Jun 9 at 8:57
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    $\begingroup$ Tangentially, Houston has an extensive walking tunnel system around key points downtown. In your world, if buildings require significant excavation anyway, tunnels have the advantage of being significantly easier to maintain and cool, they don't alter the building's aesthetic, and they leave the roadway open for unusually tall cargo. They are little additional cost to excavate if you already have to engineer flood management measures, and you can make space for storefront. $\endgroup$ – Sean Boddy Jun 9 at 12:39
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    $\begingroup$ Places in "warm" climates usually have seasons with unbearable weather too. Arizona or Texas in the summer can be just as unpleasant outside as Minneapolis in the winter. $\endgroup$ – Seth R Jun 9 at 15:14

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Reasons:

  1. Skyways can be airconditioned and so cooler
  2. Skyways can be sheltered from rain and so more pleasant
  3. Skyways are safer than walking on the streets, where the cars are; separation of cars and pedestrians means fewer collisions.
  4. Cold-city emulation: doing without a skyway makes your city look poor. Possibly quaint as well. All the prosperous modern cities have skyways!
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    $\begingroup$ Having lived in Rio for 7 years, I'd give an arm and a leg for a skyway in downtown Rio, especially in summer $\endgroup$ – Juliana Karasawa Souza Jun 9 at 6:01
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    $\begingroup$ I would also add that it can increase traffic density in the same area, as you're using also vertical travel instead of a 2D approach to most traffic. In addition, this can reduce the strain on existing traffic, as you need less points where traffic crosses each other. No traffic lights, roundabouts or the like that slow down traffic, increase chance of accidents, etc. It is done in most economically well cities with traffic already. Trains go underground/on raised ground/bridges to reduce crossings and increase overall traffic flow. $\endgroup$ – Trioxidane Jun 9 at 8:52
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    $\begingroup$ @stackoverblown This is world building 😉 As a world building excuse point 4 totally works if in your world most cities are cold. World building is all about finding decent excuses. Obviously in our world skyways didn't end up getting used (a lot) even when 1 and 2 applied. $\endgroup$ – David Mulder Jun 9 at 14:43
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    $\begingroup$ (As elsewhere noted...) 5. Skyways can be really important if you have lots of tall buildings close together. The difference in time it takes to walk straight from A to B vs. going down 50 stories, crossing at street level, then going back up 50 stories, can be quite non-negligible. $\endgroup$ – Matthew Jun 9 at 14:56
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    $\begingroup$ Exactly how hot is the climate? If it's a desert city, skyways provide a nice way to get around during sandstorms, when it might be rather unpleasant outside. $\endgroup$ – anaximander Jun 10 at 12:08
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Choosing an answer from the real world: because you have a lot of pedestrians, and without the elevated walkways they would effectively block all motor traffic.

This is the main reason for Tokyo's many elevated walkways. And Tokyo, despite having a "warm" climate and little snow, has a lot of them -- between buildings, paralleling streets, providing bridges over street crossings.

The walkways are also built for aesthetic reasons, either architecture or as parks, but the primary reason for them is to allow copious pedestrian traffic (Shibuya Crossing alone has 3000 pedestrians/minute) to not conflict with motor traffic.

So, if you just make your city sufficiently dense and pedestrian/transit oriented, you don't need a lot of additional justification.

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    $\begingroup$ Hong Kong has something similar. $\endgroup$ – DJClayworth Jun 9 at 19:53
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Tax Purposes

In the UK many university buildings are joined by this sort of walkway. The reason is that, for a while, joining two or more buildings this way allowed them to be counted as a single building for tax purposes.

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Many reasons, but the most important one is :

Because architecture is not all about pure practicality

It's a very subjective reason, yet, it's true enough. Not all architecture is about being absolutely practical, even though contrary to pure art they have to be to some extent. For instance, why would you need a so high roof in the entrance of many ever-so stylised buildings? There are no furniture nor giant people who will need such height, yet people tend to like them for the sake of not being cramped up in a small place.

On a broader scale, an architect will want (or not) to build such a bridge for the same reasons. For instance, for a Megacorp building with its key letter "H", one could want to play with the bridge to form the letter. Even broader, bridges such as these could be seen as the latest trend in urbanisation, and to keep up with it they decide to build many of them.

Know that in urbanisation, a lot of factors are taken into account. For a concrete example, in my city, there was a choice which was given to the town council : An upcoming subway was going to go right through the middle of the city, and they had to choose whether it should be built under or above the ground. While the project was more cumbersome and costly if built underground, it was built below anyway because it was much, much more pleasing to the eyes and didn't cut the city in half. You can see these underground and above stations photos to give an idea of how big the difference was.

My point here is that a lot of architecture choices are also about visual appeal. And if roofed pedestrian bridges are pretty at some place, then I don't see why an artistic architect wouldn't suggest it in the district's rehabilitation plan! At worst, air refreshers such as fans or air conditionners will be needed, which will only add a minor cost to the project.

Other specific, minor reasons in disorder

  • Because you cannot build underground passageways, such as when subway rails go along the road.
  • Because it's warm, but also overly humid. What's worse than feeling your sweat over your transpiration? It's to be drenched to a mossy hell by near constant rains.
  • Because you can quickly travel between high-floors of megastructures without burdening elevators and stairs.
  • Because it's a high-tourist pathway, and it gives you a panorama of the city, without strong winds.
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Off the top of my head, I can think of 2 reasons:

  1. Keep foot/pedestrian traffic off the street, therefore increasing pedestrian safety
  2. As someone who lives in a hot, humid climate, don't underestimate a nice air-conditioned tunnel. I have personally driven further to go to an indoor shopping centre than an outdoor just for the air-conditioning.

Additional 3rd point: it's your story and you want them.

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Scale

Many of the buildings of your city are so tall, it is considerably time consuming to travel down to ground level, then back up to the appropriate level in the new building. So relatively high-level skyways link many of the buildings, and of course skyway floors are where shops, bars etc congregate.

You could even add some kind of class stratification to the different "tiers" of pedestrian links etc, or note how certain higher levels were "islands" that were too far from other clusters of tall buildings to link, so you had to go back to a lower level for the connection if it suits your story.

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Mosquitos, perhaps carrying something unpleasant. Skyways allow people to get around without getting bitten.

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    $\begingroup$ @ARogueAnt. The question was "why would a city have skyways", not "who would build a skyway". I think anybody who's lived in an area with mosquitos should be able to understand the possible benefits of getting from A to B without going out in the open. $\endgroup$ – Geoffrey Brent Jun 10 at 5:10
  • $\begingroup$ @ARogueAnt. I am honestly bewildered at the idea that this needs expansion to be understood, but okay, done. $\endgroup$ – Geoffrey Brent Jun 10 at 5:38
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Airco combined with shorter travel

Assuming a hot city maybe people want to walk in nice air-conditioned skyways. Also it would probably be better from an energy conservation view than constantly having to open the door and let the heat in.

Besides, it might just be shorter and reduces traffic congestion. As your picture nicely shows you can easily walk above traffic straight to where you want instead of having to go down extra stairs and wait for traffic to go to the other building.

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Safety

It may be unsafe, unpractical or unpleasant for pedestrians to travel outside on foot. Some possible reasons:

  • The city is a sort of Venice, with canals instead of roads. Sidewalks may be too narrow, or absent, of perhaps the city doesn't have enough bridges so that skywalks are preferred.
  • Too much car traffic, making it dangerous to cross streets.
  • The streets are full of robbers and pickpockets. Access to the skywalks is regulated, and the riff-raff are kept out.
  • The buildings in the city are very tall, and the streets narrow. Hardly any sunlight ever reaches down to street level, turning the streets into an eerie place noone wants to be.
  • The smell outside is bad. Be it from car traffic, a nearby factory, or the scores of farm animals in the streets.

Prestige

Skywalks are for the elite. You need to pay a large sum of money to gain access to the Skywalks, with its luxury shops, fine dining restaurants and exclusive night clubs, which makes the inhabitants of the city covet access passes. If you have to walk outside, you certainly can't be a member of the country club!

Segregation

The green-skinned people praying to Boreas use the skywalks, while the blue-skinned people praying to Eurus must keep to the outside streets.

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  • $\begingroup$ I like the Venice idea! Maybe this city is in a swamp, or out at sea. $\endgroup$ – Willk Jun 9 at 4:01
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History: Your skywalk wasn't always a skywalk

Most cities tend to see a substantial change in ground level over time.

Usually, this tends to raise ground level. For example, when we repave roads (for example), we just layer the new pavement on top.1

But it can run the opposite way too: when we extract wellwater to drink, the ground (without water underneath to support it) subsides. Jakarta is subsiding so fast that Indonesia has changed the capital to another city;2 Bangkok is subsiding slower, but still noticeably. If your buildings are all anchored in bedrock that is not susceptible to the same subsidence trends, what was the first floor may become elevated.

Ground level shifts don't have to be a "natural" process either. Mid-20th century urban planners often built road trenches to avoid slowing their streets down with pedestrian crossings if they didn't have the money for a full tunnel. Before them, railways could build similar trenches (although they usually sprung for tunnels, since the ventilation cost differential was smaller). Maybe your city originally trenched its main thoroughfares…and then kept on trenching more and more streets. Eventually, the sidewalks outside the trench became a skywalk.

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A real world example: A former employer of mine was headquartered in a warm locale that rarely ever saw snow/ice. Their campus had a dozen different buildings, all 4-16 stories tall and connected by skyways. The skyways made a massive difference and made life significantly easier.

The obvious benefit is that they provided protection against rain. Some less-obvious benefits:

  • Skyways allowed us to move machinery between buildings without exposing it to the elements. Pushing hand carts full of equipment across a public road is also dangerous, difficult, and exposes you to risk of theft. This really applies to anything that needs to be kept in a controlled environment (some hospitals use skyways between buildings for this reason).
  • Buildings connected by skyways act as if they were one single building. Some buildings didn't have certain necessary features (like freight elevators), but that wasn't really a problem if you were connected to a building that had one.
  • Similarly, skyways greatly simplified our security. Instead of having to check in and out every time you left or entered a building, you could have a central security checkpoint and then move between buildings freely. It's like a single, sealed ecosystem.
  • One of our skyways crossed over private roadways that were used by heavy equipment and trucks going to and from our loading docks. Pedestrians were forbidden in this area for safety reasons. Walking around the loading area at ground level would take around half a mile, but the straight-line skyway could get you there in 1/10 the distance.
  • Travel time between buildings can be significantly reduced with skyways vs. ground travel. At one point I worked on the 12th floor and had a regular meeting on the 11th floor of the next building over. Taking three flights of stairs down, a skyway over, and two flights back up saved me 10 minutes each direction over taking the elevator all the way down, walking across the street, and taking the elevator all the way back up. Plus, my chances of getting hit by a car were reduced by more than 50%.
  • The back half of our campus dropped off in elevation by a noticeable amount. Walking between buildings at ground level meant going up or down a hill. A skyway can connect two floors located at similar elevations, even if it's the second floor of one building and the third floor of another. Once you're inside the building, you can't even notice there's a hill.
  • You can build skyways without being restricted by the way ground-level transport is designed. Imagine a standard four-way intersection with a building on each corner. Skyways can connect two buildings at a diagonal, or you can connect three buildings in a "Y" shape. Your above-ground and ground-level transport systems can have absolutely nothing in common.
  • There was a bit of a prestige element to it as well. Anyone driving by can see people constantly moving about through the skyways. The common reaction is one of "wow, look how busy that company is, they must be doing well". Completely fallacious reasoning, but a natural assumption nonetheless.
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  • $\begingroup$ "Skyways allowed us to move machinery between buildings without exposing it to the elements. Pushing hand carts full of equipment across a public road is also dangerous, difficult" - oh yeah, I was thinking about reasons while reading q and a's and was thinking, in my city there is basically none of those(if any at all), if we do not count road crossing under and over ground. But then I remembered that at industrial building/complexes/territories it is a regular thing, pretty much for this very reason. Other points good as well - solid answer $\endgroup$ – MolbOrg Jun 13 at 8:45
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Could be justified in any area with a lot of tall buildings, where a lot of people want to get from one building to another without having to travel all the way to the ground and all the way up the other side.

But as an alternative answer, a powerful Skyway developer happened to have politicians in his pocket or powerful lobbyists and managed to convince the right people that the city needed them (whether or not it actually does)

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There is a very simple answer to this that I didn't see mentioned anywhere else, so I thought I'd add it even though an answer has already been accepted. The city I live in (Cincinnati, Ohio) used to have a very extensive Skywalk or Skyway. From the 1960's through the 1980's indoor shopping malls became very popular all over the United States. Traditional store fronts on the street simply went out of style in favor of air conditioned indoor shopping malls. Cities that wanted to capitalize upon this trend started building Skywalks/Skyways throughout their cities so they could compete with the malls at attracting both retail and food service businesses as well as the customers. Today the Skyways are less popular and cities that don't have a good reason to keep them have been removing them because the trend has gone back to strip malls and street front stores while the indoor shopping malls are in decline.

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Acid rains

Your city has a warm nice climate... Too bad that the local ecology is basically nonexistent since it's in a quite industrialized area combined with bad local geographical features amplifying the problems. You REALLY don't want to get soaked in local rain, and some smog variations crop up from time to time as well. Skyways help reduce your exposure to outside conditions.

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Tall skyscrapers are just a thing in your world. Be it residential, offices, educational, and so on, and so on; everything is a skyscraper!

Naturally, the rich wants to be in the highest positions available. They can see the view (though perhaps I imagine it's only natural background overlaid with views of spikes of concrete tops on the lower half field of vision). They want to be symbolically higher and more esteemed than the commoners. They want sunlight, because in the lower levels the other buildings block it from your windows. They want fresher air when they open their windows because ground level air just sucks (it's so humid and damp, ew) and is full of unpleasant motor smells.

The next thing they want is to avoid ground level as much as possible.

They want their kids to enter the prestigious school next building but don't want the hassle of descending so far to the street level just to cross and to ascend again. So skyways are constructed.

They want to visit their fellow established members of society but don't want to meet the commoners. So skyways are constructed.

The streets are dangerous because there resides the poor, the disadvantaged, and those who have nothing to lose when they commit crimes, so skyways are constructed.

Soon after, the middle class who resides in a bit lower stories than the rich wants to have the skyway for the same reasons. They distance themselves from the peasants that live near the street.

Voila, your city now has skyways for very sinister reasons.

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Hundreds of years ago, your city was founded in the northern prairie. The gateway to the northwest, it thrived - but in the winter, temperatures rarely rose above freezing, often falling between -15C and -5C. To deal with these cold temperatures, the city built many skyways between its downtown buildings, allowing its citizens to cross between them in safety and even comfort during the long dark cold winters.

As the city grew, so did the rest of its civilization, discovering fossil fuel deposits and combusting them to fuel its economy. Over the centuries the average temperature warmed, changing local climates all across the planet. People moved from less comfortable places to more comfortable places, and as the temperature inched upward, your city thrived. As the gateway to the northwest, your city was positioned between the old great cities of the south and new farmland, resources, and trade routes unlocked from the thawing tundra.

Now the long dark winters are decidedly less cold; the temperature rarely falls below freezing, usually between 2C and 12C. People stroll about in comfort and when there is a dusting of snow, highways clog, schools close, and everybody generally loses their minds.

But the hallmark of the city's ancient architecture remain. Long a symbol of your city, both the old skyways and the newer ones have been meticulously maintained. Though they are often built open to the air now, with cool breezes blowing through and people often taking dinner on the wide new ones, the skyways still allow people to walk in comfort from skyscraper to skyscraper in the city core.

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Security

It's a niche application, but you will often find skyways linking courthouses to the county jail (not prison).

Reduces risk of prisoners escaping while being moved to/from court appearances since there is no external ground transport involved-- it's a literal pipeline.

Climate is not a factor; both Seattle and Atlanta's courthouses have skyways, as does the Doge's Palace in Venice (famously known as the "Bridge of Sighs").

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Microscopic Pollutants:

These could be chemical, biological, nuclear or rogue nanotech floating around the atmosphere and commuters might want to keep their exposure to these agents as reduced as possible

Info Safety:

Perhaps the scenario is a post-apocalyptic, post-war or cyberpunk type of environment where there is a strong preference from a land faction toward guarding from satellite or environmental observers

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The only thing I really have to add that wasn't in the previous sections is that in minecraft skyways are more common. this is because not having to terraform the ground saves time (and creating structures that would otherwise be ludicrously unstable are not due to the rarity of block gravity) making it so building in such a way spends less time, which is compounded further when in the late game flying becomes available.

In short civil engineering is hard and terraforming takes time, so that if you can avoid the latter easier than doing the work then it's go time.

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